Download Effects of electric and domestic circuits

Effects of an Electric Current and
Domestic Circuits
Chapter 24
Heating Effect of Electric Circuits
• It was James Watt who experimentally
investigated the effects of the heat, W, from a
current carrying wire. It was found that the
following equation may be used to find W:
2
W  I Rt
• where W = amount of heat energy given
I = current through the wire
R = resistance of the conductor
t = time that the current flows for
• Joule’s law states that the rate at which heat is
produced in a conductor is directly proportional
to the square of the current provided its
resistance is constant:
2
PI
• By dividing both sides of the heat effect
equation by ‘t’ we find:
2
PI R
• And by using Ohm’s law we get:
P  IV
• The advantage of using high voltage in
transmission of electricity.
The Chemical Effect of an
Electric Current
• An electric current may cause a chemical
reaction when passed through a liquid,
known as electrolysis. Applet…
• The liquid in which the current is passed
is called the electrolyte, the plates that
are in the electrolyte are called
electrodes, the positive electrode is
known as an anode, the negative
electrode is the cathode. The container,
electrodes and electrolyte are known as a
voltameter.
• Electrodes that are involved in the
chemical reaction are called active
electrodes, those that do not are called
inactive electrodes.
• Examples of an electrolyte would be water
with an acid, base or salt in it (i.e. a
solution), or an ionic compound in it’s
molten state.
• An ion is an atom or molecule that has
gained or lost one or more electrons.
• The charge carriers in an electrolyte are
positive and negative ions.
Applications of the Chemical Effect
• Electroplating, covering one metal with a
thin plating of another.
• Extraction of minerals from their ores.
• In electrolyte capacitors electrolysis is
used
• To purify metals.
Relationship between current and
voltage for different conductors
A metallic conductor
• Assuming we have
constant temperature, the
resistance of a conductor
will not change as current
increases, so we get a
straight line graph through
the origin, obeying Ohm’s
law.
• Charge carriers in a metal
are electrons.
A filament bulb
• As the voltage across the
filament increases, so does the
current and in turn the
temperature. Since resistance
α current, resistance increases.
So as the bulb gets hotter a
given increase in V doesn’t
produce an increase of I when
it was colder.
• Charge carriers in a filament
bulb are negative electrons.
A semi conductor, e.g. a
thermistor
• As the p.d. across the
semiconductor is increased, the
current increases, and in so
doing the semiconductor gets
hotter. This produces more
holes and electrons for
conduction and the resistance
drops. Thus a further increase
in V produces a larger increase
of I when it was cold.
• Charge carriers are negative
electrons and positive holes.
Active electrodes
Inactive electrodes
Electrolytes/ionic solutions
As p.d. increases so will
current. Active electrode takes
part in the chemical reactionalso obey Ohm’s law and the
graph is linear through the
origin. If the electrodes are
inactive, the voltameter
behaves like a cell and has an
emf that must be overcome
before current will flow.
Charge carriers are positive
and negative ions.
E.g. neon lamps.
A Gas
An example would be a discharge
tube.
In region OA the positive ions in the
tube are attracted to the negative
electrode and the electrons move
towards the positive electrode once a
p.d. is applied, as number of ions
crossing the tube increases so does
the current.
In region AB all the ions in the tube
cross without recombination so no
increases in current.
In region BC, voltage increases to a
stage where collision between fast
moving ions and electrons produces
more ions.
Charge carriers are positive ions,
negative electrons and a few negative
ions.
A Vacuum
No charge carriers in a
vacuum, however if the
cathode is heated
sufficiently electrons will be
produced by thermionic
emission. A certain voltage
is reached where all the
electrons from the cathode
are carried across the tube
and the curve flattens out.
Why is electrical energy
transmitted using high voltage
power lines?
• We have seen that the heat produced by an
electric current is given by the formula
P = I2R.
• This means that a large current will produce a
lot of heat energy. This is a waste of energy
and therefore should be minimised if possible.
• When power is transmitted through many
kilometres of cable, the resistance (R) of the
cable becomes significant. The power loss in
the cable due to heating is given by I2R.
Therefore, for a given cable resistance, the
smaller the current, the smaller the power
loss. Remember: P = VI
• If the power, P, is delivered at a voltage, V,
then the current through the cable is given by
I=P/V. The power loss = I2R = (p/V)2R.
Therefore, the larger the voltage V, the smaller
the power loss and so electrical power is best
transmitted at high voltages. In practice we
make the current very small by making the
voltage very large (e.g. 300 kVolts) when
electrical energy is being carried across the
countryside.
Transformers are used to change the
voltage up or down
Transmission
Where does it come from?
• Electricity generated from renewable energy
(normalised) reached 19.6% of
• gross electricity consumption (RES-E) in 2012.
The national target for 2010
• was 15% of electricity consumption generated
by renewables and the EU
• target for Ireland was 13.2%. Ireland’s target
for 2020 is 40%.
Ref: SEAI.ie
VOLTAGE SUPPLY
• In a plug the live
wire is brown,
neutral is blue and
earth
is
green/yellow.
Plugs
Brown – Live wire
Blue – Neutral wire
Green/Yellow – Earth wire
Current comes in on the live
• Many electrical appliances, including cookers,
washing machines and refrigerators, have metal
cases. The earth wire creates a safe route for the
current to flow through, if the live wire touches the
casing.
• You will get an electric shock if the live wire inside an
appliance, such as a cooker, comes loose and touches
the metal casing. However, the earth terminal is
connected to the metal casing, so the current goes
through the earth wire instead of causing an electric
shock. A strong current surges through the earth
wire because it has a very low resistance. This breaks
the fuse and disconnects the appliance.
• Some appliances, such as vacuum cleaners,
hair dryers, radios and electric drills, do not
have an earth wire. This is because they have
plastic casings, or they have been designed so
that the live wire can not touch the casing. As
a result, the casing cannot give an electric
shock, even if the wires inside become loose.
These appliances have double insulation and
carry a symbol:
Fuses
• Consists of a metal wire which melts if too
large a current flows through it, breaking the
circuit and preventing damage
• The fuse is always on the live wire
• A table lamp has a power rating of 100 W. What
is the most suitable fuse for the lamp?
• The fuse in the plug of an electric kettle was
replaced with a 5 A fuse. The kettle has a power
rating of 2 kW when connected to the ESB
mains voltage of 230 V.
• Calculate the current that flows when the kettle
is first plugged in.
• Bonding
is when metal taps, metal water tanks, etc. in
a house are earthed in case they ever come
into contact with a live wire, thus nobody is
electrocuted.
• Earthing
is where an appliance is earthed so that if a
fault develops where the live wire came in
contact with the outer metal casing, for
example a metal kettle, nobody would be
electrocuted if they touched the casing.
MCBs
• Miniature Circuit Breakers
have the advantage that they
do not need to be replaced
when they trip.
• They can contain an
electromagnet and/or a
bimetallic strip.
• Either way, when the current
exceeds a preset value, the
circuit should break.
• Residual current
devices(RCDs)
• operate by detecting a
difference between the
current in the live and the
neutral-which could arise
if somebody came in
contact with a live wire.
RCDs
• Residual Current Devices operate by detecting a difference between
the current on the live wire and the current on the neutral wire.
• These should be the same, but if they are not this indicates a
problem, and so the RCD trips.
• Note that RCBs act quicker and are therefore safer than MCBs.
Domestic Electric Circuits
• Appliances that take a large current , like an
electric cooker, electric shower or immersion
heater have a separate live and neutral wire
coming from the distribution box. Such a circuit
is called a radial circuit.
• In a ring circuit, the live terminals of each
socket are connected together. Power is thus
fed along both sides of the ring to each socket.
The neutrals are also connected together and
connected back to the neutral at the
distribution box.
Radial Circuits
• Appliances that take a large current have their own
live wire going in, and neutral wire coming out of,
the appliance (from the fuse box).
• Examples would include a cooker or a washing
machine.
Ring Circuits
• In a ring circuit the live wire from each of the
sockets are connected to a common line, as is
the case with the neutral wire from each
socket (on that particular ring).
The Kilo-watt Hour
• The kilo-watt hour is the amount of energy used by
a 1000W (1kW) appliance in one hour. Remember
that:
1kW = 1000W = 1000 joules per second
and
energy=power x time
• The ESB sells electricity units called kilowatt-hours
(kWh).
No. of kilowatt-hours = No. of kilowatts x No. of units
• To find the cost we multiply the number of kWh by
the cost per kilowatt-hour/unit.
Cost = No. of kWh x price per unit
Example:
If a unit electricity costs 12c. How much does it
cost to run a 5kW electric heater for 2 hours?
No. of kilowatt-hours = No. of kilowatts x No. of
units
=
5
x
2
=
10 kWh
Cost = No. of kWh x price per unit
=
10 x
12
=
120c
=
€1.20
RMS
• The root mean square (RMS) voltage or
current is the time-averaged voltage or
current in an AC system.
• Unlike DC current and voltage, which are
constant, AC current and voltage vary over
time. This is called alternating current because
the direction alternates.
• The root mean square (abbreviated RMS or
rms) is a statistical measure of the magnitude
of a varying quantity. We use the root mean
square to express the
average current or voltage in an AC system.
• The RMS current and voltage are the peak
current and voltage over the square root of
two.
RMS CURRENT
• the root mean square of the current,
Irms= I0/√2 ,
where I0 is the peak current, in an AC system
RMS VOLTAGE
• the root mean square of the voltage,
Vrms= V0/√2 ,
where V0 is the peak voltage, in an AC system
Why not give it a go?
• The voltage in Irish homes has a peak voltage
of 325 V. What is the corresponding value of
the root-mean-square voltage?